This application is based on and claims priority under 35 U.S.C. xc2xa7 119 with respect to Japanese Patent Application No. 2001-177746, filed on Jun. 12, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to a control apparatus of an automatic transmission and a method for controlling a shift operation of an automatic transmission. More particularly, this invention pertains to a control apparatus of an automatic transmission for restraining a vehicle wheel slippage.
An engine brake system as a control apparatus of an automatic transmission is disclosed in a Japanese Patent Laid-open application 63(1988-)159149, the disclosure of which is incorporated by reference herein. The engine brake system disclosed therein is provided with a detecting means for detecting a rotational speed (rpm) of a driving wheel, another detecting means for detecting a rotational speed of a driven wheel, and an up-shifting means for up-shifting a gear stage of a transmission. More particularly, the gear stage of the transmission is up-shifted when a difference between the rotational speed of the driving wheel and the one driven wheel exceeds a predetermined value.
According to the above described engine brake system, the slipping condition can be accurately estimated based upon the directly detected difference between the rotational speeds of the driving wheel and the one driven wheel. As described above, the gear stage of the transmission is up-shifted when the rotational speed difference exceeds the predetermined value, i.e. when any one of the wheels is slipping. The vehicle engine braking force is hence reduced in response to the up-shift operation of the gear stage of the transmission. Therefore, the vehicle can be prevented from reaching a completely slipping condition even when the vehicle engine braking force becomes greater than a vertical load of the driving wheel relative to a ground so as to tend to slip the driving wheel.
However, when the gear stage of the transmission is down-shifted based upon a shift pattern immediately after the vehicle completely slipping condition was prevented from occurring by performing the up-shift operation, the engine braking force in response to the down-shift operation becomes greater than the vertical force of the driving wheel relative to the ground. Further, the driving wheel may be slipped again with a high potentiality so that the vehicle may not be stably driven.
Accordingly, the control apparatus of the automatic transmission for restraining the vehicle wheel slip by the up-shift operation are still susceptible of certain improvements with respect to stabilizing the vehicle running condition immediately after the vehicle completely slipping condition was prevented from occurring.
According to an aspect of the present invention, a control system of an automatic transmission comprises a slip detecting means for detecting a vehicle wheel slip, an up-shifting means for up-shifting a shift stage when the vehicle wheel slip is detected by the slip detecting mean, and a down-shift line setting means for setting a first down-shift line at a lower vehicle speed side than a vehicle speed of a second down-shift line which had been set before performing the up-shift operation by the up-shifting means. More specifically, the first down-shift line is newly set only when the vehicle wheel slip is not detected any more by performing the up-shift operation by the up-shifting means.
According to another aspect thereof, it is preferable that the control system of the automatic transmission further comprises a road inclination detecting means for detecting an inclination of a vehicle running road and a shift pattern switching means for switching a shift pattern of the automatic transmission corresponding to the road inclination detected by the road inclination detecting means. In such a case, the down-shift line is set by the down-shift line setting means in response to the shift pattern switched by the shift pattern switching means when the vehicle wheel slip is not detected any more by performing the up-shift operation by the up-shifting means.